1. Field of the Invention
The present invention relates to the field of yttrium aluminate garnet (YAG) particles. In particular the present invention relates to yttrium aluminate garnet particles coated with a sintering aid for use in fabricating ceramics.
2. Description of the Related Technology
Hard, strong and thermally conductive YAG particles are generally used to fabricate ceramics used in the semiconductor and optics industry. Although single crystal YAG particles are widely used to construct single crystal laser hosts, these crystals are expensive to manufacture, too small in size and do not provide sufficiently good beam quality to be used in high power lasers of greater than about 5 kW. By contrast, polycrystalline materials possess good beam quality and can be made in large sizes.
Processing YAG particles to form polycrystalline YAG ceramics overcomes many of these limitations. Traditional processing of polycrystalline YAG ceramics, however, leads to high scattering and absorption losses that are distributed in localized yet random regions. Polycrystalline YAG materials produced by this method do not posses uniform optical loss and therefore have a poor yield. Furthermore, traditional processing is expensive and inadequate for producing large sized polycrystalline YAG ceramics or materials having different shapes. Consequently, YAG materials produced by this method are unsuitable for many applications, including construction of high power lasers.
One of the primary problems with traditional processing is the difficulty involved in sintering YAG-containing materials. In general, sintering is the act of consolidating powder into a dense shape. The powder being sintered cannot melt to a great extent, although some melting of the secondary phase in the powder or surface melting is allowed under this definition. If the material melts, the process is referred to as fusion casting. Sintering, either with pressure, i.e. hot pressing, or without pressure, requires a vast amount of material transport to consolidate an aggregate of loose powder particles into a dense shape. In the case of porcelains and clay products, the secondary phases of these materials melt and bind the primary solid particles together with a glass phase; these types of systems were the first to be used due to their ease of sintering. However, advanced ceramics do not have these intrinsic sintering aids and thus sintering aids must therefore be added to the materials.
Sintering aids work in a variety of fashions. They may liquify at or somewhat below the primary material's densification temperature thereby promoting liquid phase sintering. Certain sintering aids exhibit higher solid-state diffusion coefficients than the primary material's self-diffusion. The sintering aid may conversely have a lower solid-state diffusion coefficient that prevents exaggerated grain growth and promotes grain boundary refinements and pinning. The sintering aid may also simply clean or etch the primary material's surfaces thereby enhancing solid-state diffusion.
Recently, sintering aids have been used to facilitate processing of YAG particles by enhancing densification and assisting sintering. In general, these sintering aids tend to be solid inorganic particles, such as silica. Since the YAG particles are also solid inorganic particles, the sintering aid and YAG particles must be mixed homogeneously for the sintering aid to be effective. To date this has typically been accomplished by some form of mechanical mixing. For small samples, the mixing may involve the use of a mortar and pestle. In larger samples ball milling, attritor milling, high shear wet milling, and variations or combinations of these methods may accomplish mixing. However, due to the nature of this type of particle-particle interaction, none of these mechanical mixing methods produce a homogenous mixture wherein the silica adequately coats the YAG particles. Furthermore, mechanical mixing has the added problem of contaminating the YAG with the milling material. Inhomogeneity results in areas having too much, too little or no sintering aid and causes significant problems in the fabrication of transparent ceramics, electronic ceramics, and refractory ceramics. Consequently, the ceramic materials produced from these YAG particles typically contain inhomogeneous regions as well as opaque regions having low yield that must be drilled out and removed. This can be expensive and lead to small sized ceramic products. Production of uniform and highly transparent polycrystalline YAG products therefore depends upon the uniform mixture of YAG particles and sintering aids, which as discussed above, is typically non-ideal when mechanical mixing methods are employed.
The use of solutions and suspensions including YAG particles and sintering aids for fabricating polycrystalline YAG products has also been investigated. These investigations have demonstrated that the production of a high yield and uniform ceramic depends on uniformly coating the YAG particles with the sintering aids so that the YAG particles do not substantially directly contact one another. Merely creating a solution or suspension of YAG particles and a sintering aid is insufficient to achieve this effect. Rather, further processing is required of these solutions and suspensions to produce a uniform coating.
For example, U.S. Patent application publication no. 2005/0281302 (Lee), disclose a transparent polycrystalline YAG material that may be doped with rare earth elements for use in lasing systems. The material was fabricated by dispersing YAG powders in a solution of ethyl alcohol and TEOS or colloidal silica, and the resultant suspension was dried under stirring and subsequently calcinated. The process of stirring and calcinating a YAG particle and silica precursor solution alone, however, does not produce a uniform coating of silica on the YAG particles. Although this process provides a better distribution of silica on the YAG particles in comparison to mechanical mixing, there are still areas on the YAG particles with either too much or not enough silica sintering aid. Consequently, Lee does not teach a method for producing a uniform coating of silica on YAG particles. In Lee's method, adjacent YAG particles can be directly in contact with each other.
Similarly, U.S. Patent application publication no. 2007/0182037 (Rabinovitch) discloses a transparent ceramic constructed from YAG that is produced by suspending doped YAG particles in deionized water and colloidal silica, agitating the solution and subsequently filtering out and compacting the resultant YAG particles. Rabinovitch employs a particulate silica suspension but does not indicate that the silica particles were deposited the YAG particles to establish a uniform coating. Notably, the resulting silica coating is less uniform than that taught by Lee. Rabinovitch, therefore, does not teach a method for producing an adequate uniform coating of silica on YAG particles. In Rabinovitch's method, adjacent YAG particles can be directly in contact with each other.
U.S. Pat. No. 7,449,238 (Villalobos) discloses yttria particles coated with LiF. According to Villalobos, a LiF salt is dissolved in water and caused to precipitate on the particle as the particle is dried. Unfortunately there are no commercially suitable salts that produce SiO2 without calcination at temperatures above 700° C. and long residence times measured in hours. The maximum temperatures of spray dryers are in the 500° C. range and the residence times are measured in seconds instead of hours. Although a salt could be precipitated on the particles, subsequent heat treatment at 700° C. for 2 hours to form SiO2 would produce hard agglomerates that do not sinter and would also cause spalling of the coating. Therefore, Villalobos does not provide a teaching for creating a substantially uniform silica coated YAG particle. By contrast, applying a uniform coating of silica to a YAG particle, involves a distinctly different chemical process that requires binding the silica to the YAG particle surface. Consequently, the coating method disclosed in Villablobos would not be applicable to establishing a uniform silica based coating. Additionally, Villalobos does not suggest that its coating method would be compatible with YAG particles.
Therefore, there is a need in to develop YAG particles suitably coated with a sintering aid in order to enhance the reproducibility of production of polycrystalline YAG products.
There is also a need for methods for making such coated YAG particles that do not require the advance preparation of a homogenous mixture of silica and YAG particles.